In the semiconductor industry, critical steps in the production of semiconductor wafers are the selective formation and removal of films on an underlying substrate. The films are made from a variety of substances, and can be conductive (for example metal or a magnetic ferrous conductive material) or non-conductive (for example an insulator or a magnetic ferrite insulating material).
Films are used in typical semiconductor processing by: (1) depositing a film, (2) patterning areas of the film using lithography and etching, (3) depositing a film which fills the etched areas, and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP). Films are formed on a substrate by a variety of well-known methods, for example physical vapor deposition (PVD) by sputtering or evaporation, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and electro and electroless plating. Films are removed by any of several well-known methods, for example chemical-mechanical polishing (also known as CMP), reactive ion etching (RIE), wet etching, electrochemical etching, vapor etching, and spray etching.
One of the multi-level interconnection processes for metallization to form circuits involves: (1) depositing a seed and/or barrier layer and a conductive film on top of a lithographically defined substrate, so that the conductive film fills in interlevel connections known as vias, or intralevel lines in a dielectric as well as covers the dielectric film, (2) chemical-mechanical polishing (CMP) down to the interface between the barrier layer and the underlying insulating film such as silicon dioxide, leaving the conductive film only in the vias/lines to form wires, (3) depositing another layer of insulating film, (4) planarizing down to a specified thickness, (5) lithography and patterning, and (6) repeating steps (1)-(5). The process is repeated by forming, patterning, and selectively removing films in order to manufacture the desired semiconductor circuit wiring.
It is extremely important with formation and removal of films to stop the process when the correct thickness has been added or removed (the endpoint has been reached). With CMP, a film is selectively removed (i.e. the portion of the film not in the vias or lines) from a semiconductor wafer by rotating the wafer against a polishing pad (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a chemically reactive slurry. Overpolishing (removing too much) of a conductive film results in increased circuit resistance and potential scrapping of the wafer. Since many process steps have already taken place prior to CMP, scrapping a wafer during the formation of the metal interconnections can mean a significant financial loss. Underpolishing of a conductive film (removing too little) on the other hand leads to a failure in isolating circuits and results in electrical shorts, which leads to rework (redoing the CMP process) which raises the cost of production. Various methods have been employed to detect when the desired endpoint for removal has been reached, and the polishing should be stopped.
The prior art methods for CMP endpoint detection involve the following types of measurement: (1) simple timing, (2) friction or motor current, (3) chemical analysis of the slurry, (4) capacitive, (5) optical, (6) acoustical, and (7) conductive. These prior art methods each have inherent disadvantages such as inability for real-time monitoring, the need to remove the wafer from the polishing apparatus (not in-situ), or a lack of sensitivity.
The simple timing method gives large errors because it is affected by thickness variations of the film and polish rate variations caused by composition of the slurry, pressure of the wafer against the pad, type of pad, and relative rotational speeds. Monitoring the motor current change due to the change in friction produced between the wafer and the pad only provides a resultant value for the variations and provides indirect wafer monitoring at best, with average values for the wafer. Chemical analysis of the slurry requires transporting the slurry from the polishing pad to the analysis location, as well as the use of expensive instrumentation such as inductively coupled plasma (ICP) for atomic emission spectroscopy and does not provide true real time response. Capacitive measurements embed sensing elements in the polishing table below the polishing pad and thus do not provide a continuous and reliable measurement of the change during removal. Capacitive measurements are also especially ill suited for metal films on top of multiple levels of metal interconnections. An optical method has also been used, but requires that the process be interrupted from time to time for measurement of the reflectivity or thickness change. Acoustical methods have also been proposed, however no encouraging data is available so far. Conductive methods monitor current flowing from electrodes embedded in either the polishing pad or the polishing table through the wafer. This type of method requires some kind of direct contact between the electrodes and the wafer surface as well as their exposure to the corrosive slurry and contact with the polishing pad, which can lead to contamination of the pad and possible scratching of the wafer. Results have so far not been positive with this approach.
Non-CMP specific methods have been used to monitor metal articles, but are not suited for in-situ monitoring of the change in thickness of a film on an underlying body. For example, the use of inductive probes to determine breaks in metal articles is known in the art. In IBM Technical Disclosure Bulletin Vol. 9 No 4 Sep. 1966 entitled "Detecting Undesired Breaks in Metal Ladders" by Soychak, a U-shaped core pair is positioned with one core on each side of a ladder, with the top core connected to an oscillator. A continuous metal path around the core pair loads the pair and tank coil of the oscillator and causes its signal level to drop. A break is indicated when there is no loading of the oscillator circuit. This apparatus cannot discriminate film changes from surrounding metal, and when combined with the fact that the cores are on both sides of the object, cannot be used to monitor in-situ the change in thickness of a film, especially in a CMP process.
A technique to measure the thickness of coatings on metal objects is also known. For example, in U.S. Pat. No. 4,715,007 to Fujita et al entitled "Instrument for Measuring Film Thickness", a probe made of an iron core with a coil wound around it is pressed against an iron material whose surface is coated with an insulating film. The change in current as the probe approaches the iron material indicates the thickness of the film. Fujita's apparatus cannot be used to monitor in-situ the change in thickness of a conductive film, and therefore cannot be used to monitor a CMP process for a conductive film. It measures changes in the field extending across the gap between the metal and the probe. This type of sensor cannot be embedded in a surrounding metal container because the leakage fields around the gap would be affected. The field will also be affected by other metals in the area of the stray flux, such as other metal nearby the film to be measured.
Another example is U.S. Pat. No. 3,626,344 to Shaternikov, et al. titled "Eddy Currents Transducer for Electrical Devices to Control Coating Thickness and Surface Profile of Metal Articles," Shaternikov teaches an enhancement in using eddy currents to monitor and control the surface profile of articles featuring both complex configurations and small radii of curvature. Shaternikov's apparatus consists solely of an inductance coil that has been enhanced to strengthen the magnetic field that is generated. His apparatus is simply a transducer to enhance the sensitivity in examining coatings on metal articles. It cannot discriminate between a conductive film in the presence of other metal, and therefore it cannot detect changes in such a film. Therefore it does not have the in-situ monitoring capability to detect changes in films, nor is it suitable for monitoring a CMP process for the same reasons as the above references.
What is not known in the prior art is the in-situ and real time contactless monitoring of the change in thickness of a film on an underlying body which can be any material, including a conductive substrate. Thus, it is an object of the present invention to provide a method and apparatus for in-situ monitoring of the change in thickness of a film on an underlying body.
Another object of the present invention is to provide for in-situ monitoring of the removal of a conductive film from a semiconductor substrate.
Another object of the present invention is to provide for in-situ monitoring of the removal of a film from a semiconductor substrate by chemical-mechanical polishing.
A further object of the present invention is to provide for in-situ monitoring of the formation of a conductive film on a semiconductor substrate.